This invention relates to a needleless hypodermic injector, and more particularly a spring-powered needleless injector.
There is a widespread interest in the use of needleless injectors for the injection of medicines into the body of both humans and animals. A needleless jet injector makes a substantially smaller hole than does a needle, and consequently needleless injection is less painful than injection with a needle. Moreover, for people requiring frequent injections, e.g., diabetics, needleless injection has the important advantage of causing less damage to the skin and tissue. This is very significant, since the areas on the body which can be used for injection are limited.
The typical needleless hypodermic injector is adapted to discharge a liquid medicine from an orificed cylinder in the form of a minute stream or jet at two different pressure stages sequentially. The initial high pressure discharge causes the jet stream to distend the skin and force the liquid to a predetermined depth beneath the surface, depending upon the magnitude of the pressure. After the minute opening in the epidermis has been produced, the pressure of the stream is immediately reduced to a lower second stage for completing transfer of the remaining liquid from the cylinder.
Such devices have been known for more than forty years and, for instance, a typical two-stage hypodermic jet injector is described in U.S. Pat. No. 2,398,544. Many other patents have also issued over the years describing various forms of needleless injectors. Few of the devices disclosed by these patents are available in the marketplace. Many of the patented devices lack simplicity and are too expensive to produce or use.
While the concept of the needleless injector is quite simple, there have been many problems in its practical usage. Thus, the injector must be compact and light and very simple to use while, at the same time, being capable of generating very high pressures. For instance, the initial high pressure discharge is usually at a pressure of in excess of 700 kg/cm.sup.2 and the lower second stage is typically at a pressure in the order of 200 kg/cm.sup.2. If any substantial volume of medicine is to be discharged by such device, it becomes evident that a very high spring pressure will be necessary. It is also important that a substantial pressure be maintained throughout the entire injection.
An important advance in needleless jet injectors is described in Lindmayer, et al, U.S. Pat. No. 4,623,332 issued Nov. 18, 1986. This is a very compact needleless jet injector in which the spring drive comprises a series of axially aligned disc springs adapted to move an injector piston such that the discharge pressure remains substantially constant or increasing throughout the injection following the initial high pressure skin piercing stage. While that device worked quite well, it also had certain difficulties. For instance, a large number of disc springs were necessary, making the injector somewhat heavy and there were also difficulties in obtaining uniformity of compression load in the disc springs.
It is, therefore, the object of the present invention to find a modified form of spring action which will overcome the difficulties of the disc springs described in U.S. Pat. No. 4,623,332.